It's inconsistent, even at the same hotel. I've booked mulitple advance reservations at the same BW hotel, and on some stays they charged it a day or two before my stay, and on other stays the charged it upon checkout.

BW itself does not charge. The advance charge, if any, is done by the hotel. That's another reason why it's inconsistent.

(And I've seen the same inconsistency with Marriott. One hotel where I did a number of advance reservations, most stays were charged within a couple days after I made the reservation, but a couple stays were never charged in advance and only upon checkout.)